Variable rate coding for forward link

ABSTRACT

A technique for encoding a signal used in a digital communication system in which individual traffic channel data rates may be adapted to specific channel conditions. In particular, a forward error correction coding rate is adapted for individual channels while at the same time maintaining a fixed block size independent of the FEC coding rate. This allows the system data rate to adapt to the channel conditions experienced by a specific user. Thus, users experiencing good communication conditions with low multipath distortion may be allocated higher capacity, whereas users with significant multipath distortion may make use of lower rate (higher levels of coding) error codes to maintain high quality. Messages are sent from a transmitter to a receiver to inform the receiver of the coding rate implemented at any given point in time. These parameters may be adjusted independent of transmitted power level through the expedient of ensuring that size of a transmitted frame remains constant, while permitting the ability to change FEC coding rates and FEC block sizes.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/447,022, filed Nov. 22, 1999, the entire teachings of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to wireless communication systems, andmore particularly to a technique for providing variable data rateconnections over digitally encoded radio channels.

BACKGROUND OF THE INVENTION

The first generation of personal wireless communication devices, such ascellular radio telephones, operated by allocating distinct individualradio carrier frequencies to each user. For example, in an AdvancedMobile Phone Service (AMPS) type cellular mobile telephone, two 30kiloHertz (kHz) bandwidth channels are allocated to support full duplexaudio communication between each subscriber unit and a base station. Thesignals within each such channel are modulated using analog techniquessuch as frequency modulation (FM).

Later generation systems make use of digital modulation techniques inorder to allow multiple users to access the same frequency spectrum atthe same time. These techniques ostensibly increase system capacity fora given available radio bandwidth. The technique which has emerged asthe most popular within the United States is a type of Code DivisionMultiple Access (CDMA). With CDMA, each traffic signal is first encodedwith the pseudorandom (PN) code sequence at the transmitter. Thereceivers include equipment to perform a PN decoding function in such away that signals encoded with different PN code sequences or withdifferent code phases can be separated from one another. Because PNcodes in and of themselves do not provide perfect separation of thechannels, certain systems have an additional layer of coding referred toas “orthogonal codes” in order to reduce interference between channels.

In order for the PN and orthogonal code properties to operate properlyat a receiver, certain other design considerations must be taken intoaccount. For signals traveling in a reverse link direction, that is,from a mobile unit back to a central base station, power levels must becarefully controlled. In particular, the orthogonal properties of thecodes are optimized for the situation where individual signals arrive atthe receiver with approximately the same power level. If they do not,channel interference increases.

The forward link direction presents a different problem. In particular,a signal traveling from the base station to a subscriber unit mayinterfere with another signal in an unpredictable way as a result of theso-called near far problem. For example, faraway mobile units requirerelatively high power in order to be detected properly whereas close-inmobile units require lower power. The stronger signals may interferewith proper operation of mobile units located closer to the base stationwhich typically operate with lower power levels. Unfortunately, thisbehavior depends upon the specific operating environment of the mobilecommunications system, including the topology of the surroundinggeography, the juxtaposition of the subscriber units with respect to oneanother, and other factors.

In the past, it has been possible to set power levels individually tooptimize each forward link channel so that interference is minimized. Inparticular, it has been suggested that each power level can be adjustedto affect an optimum received power level at the subscriber unit whichtends to minimize interference.

In addition, coding algorithms such as forward error correction (FEC)type algorithms using convolutional, Reed-Solomon, and other types ofcodes, may be used to increase effective signal-to-noise ratio at thereceiver. While such codes do provide increased performance in terms oflower bit error rates in noisy environments, by themselves they do notimprove the difficulties associated with co-channel interference.

SUMMARY OF THE INVENTION

The present invention provides an additional degree of freedom bypermitting individual traffic channel data rates to adapt to specificchannel conditions. In particular, a forward error correction (FEC)coding rate may be adapted for individual channels. At the same time, afixed number of FEC symbols is maintained per transmitted frame,independent of the FEC coding rates and power levels. This allows adifferent FEC rate or even a different FEC code to be assigned to eachuser channel, depending upon channel conditions, without changing theeffective transmitted power levels.

For example, if the channel is experiencing relatively good propagationconditions, the FEC coding rate may be reduced and the number of inputbits per FEC frame may be increased without changing transmit powerlevels. Because the overall information rate then depends upon the ratioof the raw data rate divided by the code rate, a higher information rateis obtained without producing greater interference to other userchannels.

On the other hand, if a particular channel is in a relatively bad ormarginal transmission environment, other steps can be taken to reducethe overall information rate. Specifically, rather than increasing thepower level of the transmission, the effective FEC coding rate can beincreased, and the number of input bits per FEC frame reduced. This thenpermits the channel to be more robust without increasing the transmitpower level.

In a preferred embodiment, the FEC coding rate is changed byperiodically sending a message to the intended receiver which indicatesthe coding rate to be used in future transmissions on each givenchannel. For example, in a typical implementation, a rate message may besent on the forward link paging channel or sync channel directed to aparticular receiver.

There are several advantages to the present invention. In a CodeDivision Multiple Access (CDMA) system, especially in environments wheremultipath fading or other poor channel conditions exist, power levelsneed not be adjusted in order to optimize the overall system informationrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a high-level diagram of a wireless communication system inwhich the invention may be used.

FIG. 2 is a more detailed diagram of the forward link portions of thesystem which implements variable rate coding according to the invention.

FIG. 3 illustrates a particular set of framing circuits andcorresponding coding circuits.

FIG. 4 is a chart of bit error rate versus received signal strength asmeasured in energy per bit versus spectral noise power for differentcoding rates.

DETAILED DESCRIPTION OF THE INVENTION

Turning attention now to the drawings more particularly, FIG. 1 is ablock diagram of a system 10 for providing high speed data service overa wireless connection such as, for example, a digitally modulatedwireless service known as Code Division Multiple Access (CDMA). Thesystem 10 consists of one or more base station processors 12 andmultiple subscriber access units 14-1, . . . , 14-n, . . . , 14-m(collectively access units 14). FIG. 1 illustrates one base station 12and three access units 14 by way of example only and for ease ofdescription of the invention. The invention is applicable to systems inwhich there are typically many more subscriber units communicating withone or more base stations.

The access units 14 provide wireless data services and can connectdevices such as, for example, laptop computers, portable computers,personal digital assistants (PDAs) or the like, through base station 12to a network 15 which can be a Public Switched Telephone Network (PSTN),a packet switched computer network, or other data network such as theInternet or a private intranet. The base station 12 may communicate withthe network 15 over any number of different efficient communicationprotocols such as primary rate ISDN, or other LAPD based protocols suchas IS-634 or V5.2, or even TCP/IP if network 15 is an Ethernet networksuch as the Internet. The access units 14 may be mobile in nature andmay travel from one location to another while communicating with basestation 12.

It is also to be understood by those skilled in the art that FIG. 1 maybe similar to a standard cellular type communication system in whichradio channels are assigned to carry signals between the base stations12 and access units 14. This invention, however, applies moreparticularly to non-voice, digital data transmissions of varyingbandwidths. In a preferred embodiment, the system 10 uses code divisionmultiple access (CDMA) principles for modulating the transmittedsignals. However, it is also to be understood that the invention is notlimited to using standardized CDMA protocols such as IS-95, or evennewer emerging CDMA protocols such as CDMA-One or W-CDMA. The inventionis applicable to other multiple access modulation techniques.

In order to provide data and voice communication between the accessunits 14 and the base station 12, a limited number of radio channelresources are provided via forward communication channels 16-1, . . . ,16-n, and reverse communication channels 17-1, . . . , 17-n. Theinvention provides for precise management of a way in which thesechannel signals are encoded on an as-needed basis for each access unit12. It should also be understood that data signals travelbi-directionally across the radio channels 16 and 17, i.e., data signalsoriginating at the access units 14 are coupled to the network 15, anddata signals received from the networks are coupled to the access units14.

FIG. 2 shows certain elements of the base station processor 12 andremote access unit 14 in more detail. The base station processor 12 andaccess unit 14 communicate at least in a forward direction over one ormore of the forward link channels 16-1, . . . , 16-n. It should beunderstood that the base station processor 12 and access unit 14 mayalso communicate with one another in a reverse link direction, althoughthe details of such are not shown in FIG. 2. The principles discussedherein for the forward link 16 implementation could also be used inimplementing reverse link direction communications.

In a CDMA system, the signaling on a given forward link 16-n shares acommon radio carrier frequency and time slot with signaling intended forother forward links 16-m. Therefore, it is entirely possible that thesignaling sent over a given forward link 16-n intended only for aspecific access unit 14-n may in some way interfere with the signalingtransmitted over another forward link 16-m and intended for anotheraccess unit 14-m.

The base station processor 12 more particularly includes a controller 30and signal processing circuits which generate the various signals makingup the forward link 16 transmitted signals. These include circuits forimplementing functions such as a pilot channel 32, paging channel 34,and one or more traffic channels 36. As it is known in the art, thepilot channel 32 is responsible for generating known continuous pilotsignals that permit receiver circuits in the access unit 14 to properlysynchronize to signals transmitted by the base station processor 12. Thepaging channel 34 sends control signals to the access unit 14 to, forexample, allocate traffic channel capacity over the forward link 16. Forexample, the paging channel 34 is used to send messages to the accessunit 14-n when it is necessary to allocate a traffic channel on theforward link 16-n when messages need to be sent to the access unit 14-n.

The traffic channel 36 provides a physical layer structure for sendingpayload data over the forward links 16. In a preferred embodiment, CDMAencoding is used to define the pilot channels 32, paging channels 34, aswell as the traffic channels 36.

More specifically, the traffic channel circuitry 36 includes symbolframing function 40, forward error correction logic 42, a demultiplexer44, a summer 50, and radio frequency (RF) up converters 52.

Data which is to be sent over the forward link 16 is first fed to theframing function 40. The framing function 40 packages input payload datainto conveniently sized groups referred to as frames. The size of thesepre-encoded frames will vary depending upon the particular forward errorcorrection (FEC) coding scheme selected at any given time by the FECencoder 42. What is important is that the combination of the framers 40and FEC encoder 42 produce a fixed number of output FEC symbols in eachgiven transmitted frame.

FIG. 3 is a diagram showing how the framers 40 and FEC encoders 42 areselected in pairs to accomplish this end result. The fixed output FECframe size in the illustrated embodiment is 4096 symbols. Thisembodiment uses four different FEC symbol encoders 42-1, 42-2, 42-3, and42-4 providing, respectively, a ¼, ⅓, ½, and ⅞ rate encoding. The codingrate of each FEC symbol encoder 42 indicates the ratio of the number ofinput bits to the number of output bits. The actual codes used by theFEC encoders 42 may be any of a number of different types of errorcorrection codes such as R, thus, a higher information rate is obtainedwith higher rate FEC code.

This embodiment also uses four framer circuits 40-1, 40-2, 40-3, 40-4corresponding to the four FEC encoders 42-1, 42-2, 42-3, 42-4. Forexample, the ¼ rate encoder 42-1 requires a ¼ rate framing circuit 40-1which groups incoming bits into pre-encoded FEC groups of 1024 bits,producing the desired 4096 output symbols. Similarly, the ⅓ rate encoder42-2 requires a ⅓ rate framer 40-2 to group incoming bits intopre-encoded sets of approximately 1365 bits. The ½ rate encoder 42-3users a framer 40-3 with a pre-encoded set size of 2048, and ⅞ encoder42-4 uses a framing circuit 40-4 with the pre-encoded size of 3584 bits.

Framing circuit 40 and FEC encoder 42 thus only utilize one of thespecific framers 40-1, 40-2, 40-3, or 40-4, and one of the specificencoders 42-1, 42-2, 42-3, or 42-4 at any given point in time. Whichparticular framing circuit 40 and FEC encoder 42 is activated iscontrolled by coding rate control signal 60 input to each of the framingcircuits 40 and encoder 42. The code rate select signal 60 is generatedby the controller 30.

Returning attention now to FIG. 2, a given connection may requiremultiple traffic channels to be allocated at a particular time. Forexample, the demultiplexer 44 accepts the signal produced by the FECencoder 42 and feeds it to multiple spreading circuits 46-1 and channelmodulators 48-1, which impress not only a quadrature phase shift keyed(QPSK) modulation, but also the appropriate pseudorandom noise (PN)and/or Walsh or other coding in order to produce multiple CDMA channelsignals 49-1, . . . , 49-n. These multiple CDMA traffic signals are thensummed by the summer 50, together with the pilot channel signal producedby the channel pilot circuits 32 and the paging signal produced by thepaging channel circuit 34. The output of summer 50 is then fed to the RFup converter 52.

The controller 30, which may be any convenient suitable microcontrolleror microprocessor, also has among its software programs a processreferred to as the capacity manager 55. The capacity manager 55 not onlyallocates one or more of the channel modulators 48 to a specific forwardlink 16 traffic channel connection, but also sets the value for the coderate select signals 60. In addition, the capacity manager 55 sets powerlevels for particular forward link signals 16.

A single capacity manager 55 in a base station processor 12 may managemultiple traffic channel circuits 36, each producing a number of forwardlink signals 16. The capacity manager 55 sets the code rate selectsignal 60 according to observed conditions in a corresponding trafficchannel. These adjustments to the channel physical layer characteristicsare made preferably in response to determining a signal strength value,such as by measuring a ratio of the energy per data bit divided by anormalized noise power level (Eb/No) at the receiver.

Thus, in addition to changing the power level of the individualmodulated signals generated by the modulators 48, it is also possiblewith a system according to the invention to control the Eb/No at thereceiver by adjusting the value of code rate select signal 60 in orderto select different code rates under different conditions.

For example, if a remote access unit 14 located deep inside of buildingis experiencing particularly adverse multipath or other distortionconditions, in the past it was thought necessary to increase the powerlevel of the forward link 16-n in order to obtain an appropriatereceived signal level at the access unit 14. However, with theinvention, if a full maximum data rate is not needed, then the codingrate implemented by the FEC encoder 42 can be lowered.

And in other environments where multipath distortion is minimal, such asin a direct line of sight situation, the highest code rate generator42-4 can be selected while at the same time reducing the radiated powerlevel on forward link 16-n for that particular channel. This thereforemaximizes the available data rate for a given user while also minimizinginterference generated to other users of the same radio channel.

Thus, in environments where propagation is good, the system 10 canincrease the data rate to a given user without introducing additionalinterference to other users. However, in a bad signaling environment, anadvantage is also obtained since each particular user channel can bemade more robust without increasing its power level.

Continuing to pay attention to FIG. 2, various components of the accessunit 14 will be discussed in more detail. The access unit 14 consists ofan RF down converter 60, equalizer 62, multiple rake receivers 64-1, . .. , 64-n, multiple channel demodulators 66-1, . . . , 66-n, amultiplexer 68, an FEC decoder 70, and framing circuit 72.

The RF down converter 60 accepts the forward link signal 16-n, producinga baseband digitized signal. The chip equalizer 62 provides equalizationof individual chips of the received signal, fitting it to one of severalrake finger and interference cancellation circuits 64. These circuitscooperate with multiple channel demodulator 66 in a manner which isknown in the prior art and a strip off the CDMA encoding on eachchannel. Pilot receiving circuit 74 and paging signal receiving circuit76 similarly are adapted for receiving the pilot channel signal and thepaging signal generated by the base station processor 12. Themultiplexer 68 reconstructs signals in the situation where multipletraffic channels were allocated to the particular connection.

A controller 80 executes programs which set various parameters of thecomponents of the traffic channel circuit 58. Of particular interesthere is the fact that the controller 80 executes a management process 82which determines the coding rate select signal 84 to be sent to the FECdecoder 70.

Specifically, the coding rate selected by the FEC decoder 70 at thereceiving access unit 14 must be the same as the coding rate of the FECencoder 32 at the transmitting base station processor 12 in order forthe receiver framing circuit 72 to correctly reproduce the input datasignal. Thus, in order for the system 10 to adapt to changing conditionsin the RF link 16, it is necessary for the base station processor 12 tocommunicate this information to the access unit 14 in some manner.

For example, if it is desired to allow the coding rate to change duringthe duration of a connection, which is the case in the preferredembodiment, the paging channel 34 may initially include, during achannel acquisition sequencing, a command to inform the access unit 14not only of the different channels 36 on which it will be communicating,but also to inform it of the particular encoding rate that it will beusing. Then, as a connection remains open and coding rates that areoptimum change over time, additional control messages may be embedded inthe traffic channel itself by embedding a command message within thereceived data which is fed back to the controller 80 via a commandsignal input 86.

It should be understood that measures of link quality can also bedetermined by the controller 80 from the output signal 86 andperiodically sent back to the controller 30 in the base stationprocessor 12 via a command structure on a reverse link channel (notshown). This permits the controller 30 at the base station processor 12to appropriately set optimum FEC coding rates to be used by the FECencoder 42 and the FEC decoder 70 for particular connections.

FIG. 4 is a chart of bit error rate (BER) versus Eb/No in decibels (dB)for various combinations of framers 40 and FEC encoders 42. The legendin the graph shows the performance of different rate turbo product codesnormalized for the energy in a particular bit. For example, in a stateindicated at point A, the particular channel may be operating withapproximately ½ rate turbo product code and experiencing relatively lowbit error rate of 0.05. Without adjusting the transmit power and bymerely selecting a lower rate turbo product code, such as theapproximately ¼ rate code (indicated by the rate 0.266 turbo productcode) a state B is entered for the system in which the bit error rate ismarkedly decreased to approximately 0.0002. This is accomplished withoutadjusting the energy per bit or otherwise altering transmit power level.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. An apparatus for coding channels in a communications systemcomprising: a framer receiving bits of input payload data and groupingthe same bits into a plurality of differently-sized, pre-encoded frames;a Forward Error Correction (FEC) encoder coupled to the framer, encodingbits of the plurality of differently-sized, pre-encoded frames toinclude an error correction code such that a resulting number of outputencoded symbols in a transmitted frame remains fixed, even if a numberof bits in a pre-encoded frame changes; a modulator coupled to theencoder, modulating the encoded symbols according to a multiple-accessmodulation technique to produce a modulated encoded signal; and an RFup-converter coupled to the modulator transmitting the modulated encodedsignal over a communications link.
 2. The apparatus of claim 1, whereinthe framer comprises a plurality of framer circuits, each framer circuitgrouping bits of the input payload data into a respective one of theplurality of differently-sized, pre-encoded sets of bits.
 3. Theapparatus of claim 2, wherein one of the plurality of differently-sized,pre-encoded sets of bits is selected to produce, when encoded, a desirednumber of output symbols in an encoded frame.
 4. The apparatus of claim2, wherein the encoder comprises a plurality of FEC symbol encoders,each FEC symbol encoder coupled to a respective one of the plurality offramer circuits and encoding bits of the respective pre-encoded framesaccording to a different encoding rate, each of the coupled framecircuit-FEC symbol encoder pairs providing the same number of encodedsymbols in an encoded frame.
 5. The apparatus of claim 4, wherein aselectable one of the plurality of coupled framer circuit-FEC symbolencoder pairs is activated at any given point in time.
 6. The apparatusof claim 5, wherein the framer and the FEC symbol encoder each receive acoding rate control signal, the framer selectively activating one of theplurality of framer circuits responsive to the received coding ratecontrol signal and the FEC symbol encoder selectively activating one ofthe plurality of FEC symbol encoders responsive to the received codingrate control signal, such that the activated framer circuit and FECsymbol encoder, when coupled, provide the same number of encoded symbolsin an encoded frame.
 7. The apparatus of claim 1, wherein the FECencoder comprises a plurality of FEC symbol encoders, each FEC symbolencoder respectively encoding bits of the frames according to adifferent encoding rate.
 8. The apparatus of claim 1, wherein themultiple-access modulation technique comprises using a spreading code.9. The apparatus of claim 1, wherein the modulation technique comprisesCode Division Multiple Access (CDMA) modulation.
 10. The apparatus ofclaim 1, wherein the communications link is a wireless communicationslink.
 11. A method for coding channels in a communication system inwhich a digital signal is communicated from a transmitting station to areceiving station, the method comprising the steps of: receiving bits ofinput payload data; grouping the bits of the received input payload datainto a plurality of differently-sized, pre-encoded frames; Forward ErrorCorrection (FEC) encoding for each of the differently-sized, pre-encodedframes, the bits of the respective frame to include a respective errorcorrection code such that a resulting number of encoded symbols in anencoded frame remains constant; selecting encoded symbols from one ofthe plurality of differently-sized frames; modulating the encodedsymbols of the selected encoded frame according to a multiple-accessmodulation technique to produce a modulated encoded signal; andtransmitting the modulated encoded signal over a communications link.12. The method of claim 11, wherein the respective sizes of theplurality of differently-sized, pre-encoded frames are selected toproduce, when encoded, a desired number of output symbols in an encodedframe.
 13. The method of claim 11, wherein the encoding step comprisesproviding a plurality of FEC symbol encoders, each FEC symbol encodersencoding bits of a respective one of the plurality of differently-sized,pre-encoded frames according to a different encoding rate.
 14. Themethod of claim 11, wherein the multiple-access modulation techniquecomprises using a spreading code.
 15. The method of claim 11, whereinthe modulation technique comprises Code Division Multiple Access (CDMA)modulation.
 16. The method of claim 11, wherein the communications linkis a wireless communications link.
 17. An apparatus for coding channelsin a communications system comprising: framing means for receiving bitsof an input signal and grouping the bits into a plurality ofdifferently-sized frames; encoding means coupled to the framing meansfor respectively encoding bits of the plurality of differently-sizedframes to include an error correction code such that a resulting numberof encoded symbols in a transmitted frame remains constant, even if anumber of bits in a frame changes; modulating means coupled to theencoding means for modulating the encoded symbols by a spreading code toproduce a modulated encoded signal; and frequency-translating meanscoupled to the modulating means for transmitting the modulated encodedsignal over a communications link.